1. Field of the Invention
The present invention relates to thermal printers wherein the selective energization of heating elements causes the transfer of dye to a receiver member while minimizing line gaps.
2. Description of the Prior Art
Some thermal printer apparatus use a dye transfer process. In this process, a carrier containing a dye is disposed between a receiver, such as paper, and a print head formed of for example a plurality of individual thermal heat producing elements which we will refer to as heating elements. The receiver is mounted on a rotatable drum. The receiver and carrier are generally moved relative to the print head which is fixed. When a particular heating element is energized, it is heated and causes dye to transfer (e.g. by sublimination) from the carrier to an image pixel in the receiver. The density, or darkness, of the printed dye is a function of the temperature of the heating element and the time the carrier is heated. In other words, the heat delivered from the heating element to the carrier causes dye to transfer to an image pixel of a receiver. The amount of dye is directly related to the amount of heat transferred to the carrier.
Thermal dye transfer printer apparatus offer the advantage of true "continuous tone" dye density transfer. By varying the heat applied by each heating element to the carrier, a variable dye density image pixel is formed in the receiver.
The print head heating elements are modulated (operated) in pulse width or pulse count modes of operation. In pulse width modulation a single constant current pulse is applied to each heating element. The pulse width of a constant current pulse causes its image pixel to have a desired gray scale. Pulse width modulation varies the percentage of the line printing time that a heating element is energized and thereby varies the time that the heating element is above the dye transfer temperature. In printing images, the individual heating elements must not be allowed to overheat and sustain permanent damage. In pulse count modulation, the number of constant current pulses is varied to produce the desired image pixel gray scale.
Using high pulse rate stepping motors or dc drive motors, most thermal printing systems rotate the drum and thereby provide relative motion between the heating elements and the receiver during printing. The reasons for producing this motion are to avoid overheating, sticking of the carrier to the head and maintaining relatively smooth motion for registration control. As the dye is deposited, there is some degree of smear which is desirable, since the image is supposed to be of a spatially continuous form and some smear helps to "integrate" the image pixels to the viewer. Due to this continuous motion of the receiver and the partial power duty cycle of the heating elements, some objectionable "gaps" between image lines can often be observed, depending on the actual size of the heating elements themselves. These gaps take the form of lines transverse to the direction of receiver movement, growing in perceptibility as the image density decreases. As demonstrated in FIG. 6A and discussed later in this disclosure, dye is mostly transferred over the first half of the allocated image pixel space. As the density of image pixels decrease, miage line gaps increase and become more objectionable.
To minimize these image line gaps, the area of the heating elements can be increased to cause successive lines to overlap. At lower and lower percentages of power cycles this solution has problems. First, the power required for a heating element to reach a desired temperature will increase as its area increases. Also, increasing the area of the heating elements will reduce spatial resolution of the printed image.